My MixRGB node not working as expected
Okay, I'm looking at your .blend now.

There isn't one easy "here's the problem!" sort of thing. It's more like a series of problems. So I will try to, while attempting to keep this as brief as possible, point out each of the problems I see with your material and try to help you get the effect I think you're aiming for.

(Nice zamboni, btw!)

The first thing is, this makes no sense:



Neither does this:



Do this instead:



Another no-no...



Do this to see your material:



Here is how you can combine two images using the alpha of a PNG:

One important thing to understand about both RGB and shader mix nodes is that when something is plugged into the Fac input any white image data is considered positive (or 1 if you like binary) and any black image data is considered negative (0). All shades of gray fall somewhere in between. If you know what a PNG's alpha channel looks like you can probably imagine how this is all working now. We have taken that grayscale channel and told Cycles we want that to factor a mix of colors or shaders instead of transparency. Another important thing to understand is that whatever is plugged into the upper socket is what will mask through as 0 and whatever is plugged into the lower socket will mask through as 1. 



But wait, something's wrong...

There are no images in this .blend. Looks like you forgot to pack them before sharing. For the future, this can be done from File > External Data > Pack All Into .blend.



Let's look on Pixabay to see what image we might be able to substitute for a taillight...



A bright red magic mushroom! Perfect.

Pixabay image credit shout-out to "adege"

Make a transparent PNG, and we're up and running again.

(Providing transparent PNG below. Use responsibly.)





At this point you have all you need to show the decal on your material.

But since it is a light after all, let's take it a bit further.

Connect an Emission Shader like this so light will be emitted from only the non-transparent parts of the PNG:



And I saw you were trying to add some displacement with your original node setup, so let's add that back in the correct way.



I think you'll need to change the lighting in your scene before the bumps will be visible on your model, but you can see them in the material preview. Adjust the Noise Texture parameters to your liking. The Math Node (set to Multiply) tames the bump values.

Now everything is awesome!

Oh wait... the decal is upside-down.

We had better fix your UV map too.

Wow! That is one hell of a UV map.



Since you've already got the 2D Cursor in the right place and you have it set as your pivot point, we can just rotate this bad boy 180° and we'll be good to go.



Okay! And that concludes this answer. Phew! I think I broke a sweat writing this. Also experiencing some pupil dilation and my computer's display looks amazing; how odd. Anyway, good luck and happy blending!